What is the concept of training a neural network?
Sorry, this is a very broad area. Proper understanding of neural networks requires advanced mathematics. It's not sufficient to say "balancing of weights and biases" because most ML algorithms have weights. You seriously need to grab a book.

OCR system itself is also very broad, it includes various object recognition techniques. You haven't even mentioned what you want to detect.

If you want to study, try:


  https://github.com/Elucidation/tensorflow_chessbot


This is a well-documented OCR example for chess pieces. The project uses both regression and convolutional neural network.